The parity of the fields of a video signal enables lines to be interlaced when displayed. Hence, line 1 starts at the top left of the screen and line 313 (for a scanning frequency of 50 Hz) or line 266.5 (for a scanning frequency of 60 Hz) starts at the top center of the screen. Furthermore, the parity of the fields changes at each field for an interlaced signal.
In analog circuits, the parity is naturally managed by the position of vertical synchronization pulses, which occur either at the start of the line or at the center of the line. It is therefore not necessary to make the parity explicit, since the vertical synchronization signal directly triggers the return of the spot to the top of the screen.
But it now seems to be necessary to transcribe the conventional analog video stream to a digital stream similar to that provided by the new image sources, such as DVD and digital terrestrial broadcast via cable or via satellite. This would result in an extension of the features offered by television sets, such as improved integration which generates savings.
However, according to the standard governing this digital transcription of video images, vertical synchronization is no longer provided in the same way. Flipping of a bit representing the parity causes the spot to be returned to the top of the screen. Thus, at a scanning frequency of 50 Hz (PAL and SECAM systems) this parity bit has the value 0 for lines 1 to 312 and 1 for lines 313 to 625. For a scanning frequency of 60 Hz (NTSC system in particular) this bit has the value 0 for lines 4 to 265 and 1 for lines 266 to 3.
It is therefore important to express the parity very accurately. The positions of the vertical synchronization pulses thus assume less importance. This is because if there is an error in the parity of a field, the parity bit will not flip between this field and the previous and following fields. This failure to flip may turn out to be very detrimental since it directly affects the operation of the digital signal receivers, which then start to search for the flip. This gives rise to an image which appears to turn over on the screen, i.e., rapid vertical movement.
However, this determination is particularly difficult for noisy signals and signals from video tape recorders. In the former case, there are sufficiently distorted vertical synchronization pulses which cause the instant at which the presence of this vertical synchronization is determined to change. The parity depends only on the position of the vertical synchronization with respect to the horizontal phase. This results in a significant risk of error.
For video tape recorders the problem is the reverse. Since the signals do not generally present much noise, the position of the vertical synchronization is well determined. However, the phase of the phase-locked loop relative to the video signal may have been greatly distorted due to the presence of phase jumps a few lines before the appearance of the vertical synchronization. In these conditions, the relation between the horizontal phase and the vertical synchronization pulse is then again distorted, and there is a risk of the parity inverting.